Errno

Ruby exception objects are subclasses of Exception. However,
operating systems typically report errors using plain integers. ModuleErrno is created dynamically to
map these operating system errors to Ruby classes, with each error number
generating its own subclass of SystemCallError. As the
subclass is created in module Errno, its name will start
Errno::.

The names of the Errno:: classes depend on the environment in
which Ruby runs. On a typical Unix or Windows platform, there are
Errno classes such as Errno::EACCES,
Errno::EAGAIN, Errno::EINTR, and so on.

The integer operating system error number corresponding to a particular
error is available as the class constant
Errno::error::Errno.